System for detecting anomalies and/or features of a surface

ABSTRACT

A cylindrical mirror or lens is used to focus an input collimated beam of light onto a line on the surface to be inspected, where the line is substantially in the plane of incidence of the focused beam. An image of the beam is projected onto an array of charge-coupled devices parallel to the line for detecting anomalies and/or features of the surface, where the array is outside the plane of incidence of the focused beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/949,078,filed on Sep. 24, 2004, which in turn is a continuation of applicationSer. No. 10/452,624, filed May 30, 2003, which is a continuation ofapplication Ser. No. 08/904,892, filed Aug. 1, 1997, now U.S. Pat. No.6,608,676, which applications are incorporated herein in their entiretyby this reference.

BACKGROUND OF THE INVENTION

This invention relates in general to surface inspection systems, and inparticular, to an improved system for detecting anomalies and/orfeatures of a surface.

The need to detect anomalies of a surface such as those on the surfaceof a semiconductor wafer has been recognized since at least the early1980's. In the article “Automatic Microcircuit and Wafer Inspection inElectronics Test,” May 1981, pp. 60-70, for example, Aaron D. Garadiscloses a wafer inspection system for detecting whether microcircuitchips are placed upside down or not and for detecting flaws. In thissystem, a light beam from a laser is passed through a beam expander anda cylindrical lens having a rectangular aperture, where the lens focusesthe beam to a narrow line of laser light transverse to the incidenceplane of the beam to illuminate the wafer surface. It is stated in thearticle that the smallest defect the system can reveal is less than 10microns wide.

The size of semiconductor devices fabricated on silicon wafers has beencontinually reduced. The shrinking of semiconductor devices to smallerand smaller sizes has imposed a much more stringent requirement on thesensitivity of wafer inspection instruments which are called upon todetect contaminant particles and pattern defects as well as defects ofthe surfaces that are small compared to the size of the semiconductordevices. At the time of the filing of this application, design rule fordevices of down to 0.2 microns or below has been called for. At the sametime, it is desirable for wafer inspection systems to provide anadequate throughput so that these systems can be used for in-lineinspection to detect wafer defects. One type of surface inspectionsystem employs an imaging device that illuminates a large area andimages of duplicate areas of surfaces, such as a target area and areference area used as a template, are compared to determine differencestherebetween. These differences may indicate surface anomalies. Suchsystem requires significant time to scan the entire surface of aphotomask or semiconductor wafer. For one example of such system, seeU.S. Pat. No. 4,579,455.

U.S. Pat. No. 4,898,471 to Stonestrom et al. illustrates anotherapproach. The area illuminated on a wafer surface by a scanning beam isan ellipse which moves along a scan line called a sweep. In one example,the ellipse has a width of 20 microns and a length of 115 microns. Lightscattered by anomalies of patterns in such illuminated area is detectedby photodetectors placed at azimuthal angles in the range of 80 to 100°,where an azimuthal angle of a photodetector is defined as the angle madeby the direction of light collected by the photodetector from theilluminated area and the direction of the illumination beam when viewedfrom the top. The signals detected by the photodetectors from a regionare used to construct templates. When the elliptical spot is moved alongthe scan line to a neighboring region, scattered light from structureswithin the spot is again detected and the photodetector signal is thencompared to the template to ascertain the presence of contaminantparticles or pattern defects. While the scanning beam scans across thesurface of the wafer, the wafer is simultaneously moved by a mechanicalstage in a direction substantially perpendicular to the sweep direction.This operation is repeated until the entire surface has been inspected.

While the system of Stonestrom et al. performs well for inspectingwafers having semiconductor devices that are fabricated with coarserresolution, with a continual shrinking of the size of the devicesfabricated, it is now desirable to provide an improved inspection toolthat can be used to detect very small anomalies that can be difficult todetect using Stonestrom's system.

In the wafer inspection system where a light beam illuminates a smallarea of the surface to be inspected, such as those by Stonestrom et al.and Gara described above, the size of the illuminated spot affects thesensitivity of the system. If the spot is large relative to the size ofthe defects to be detected, the system will have low sensitivity sincethe background or noise signals may have significant amplitudes inrelation to the amplitudes of the signals indicating anomalies withinthe spot. In order to detect smaller and smaller defects, it is,therefore, desirable to reduce the size of the illuminated area on thewafer surface.

However, as the size of the illuminated area is reduced, throughput isusually also reduced. In addition, a smaller spot size imposes a muchmore stringent requirement for alignment and registration. As discussedabove, in many wafer inspection systems, it is common to perform atarget image to a reference image comparison for ascertaining thepresence of anomalies. If the area illuminated is not the intendedtarget area but is shifted relative to the target area, the comparisonmay yield false counts and may become totally meaningless. Such shiftingof the image relative to the intended target area is known asmisregistration.

Misregistration errors can be caused by misalignment of the illuminationoptics due to many causes such as mechanical vibrations, as well as bychange in the position of the wafer such as wafer warp or wafer tilt orother irregularities on the wafer surface. For this reason, a waferpositioning system has been proposed as in U.S. Pat. No. 5,530,550 toNikoonahad et al. In this patent, Nikoonahad et al. propose to use thespecular reflection of the scanning beam and a position sensitivedetector for detecting the change in height of the wafer and use suchinformation to alter the position of the wafer in order to compensatefor a change in height or tilting of the wafer surface.

While the above-described systems may be satisfactory for someapplications, they can be complicated and expensive for otherapplications. It is, therefore, desirable to provide an improved surfaceinspection system with improved sensitivity and performance at a lowercost that can be used for a wider range of applications.

SUMMARY OF THE INVENTION

One aspect of the invention is directed towards a method for detectinganomalies and/or features of a surface, comprising focusing a beam ofradiation at an oblique incidence angle to illuminate a line on asurface, said beam and a direction through the beam and normal to thesurface defining an incidence plane of the beam, said line beingsubstantially in the incidence plane of the beam; and imaging said lineonto an array of detectors, each detector in the array detecting lightfrom a corresponding portion of the line.

Another aspect of the invention is directed towards a method fordetecting anomalies of a surface and/or a surface feature, comprisingfocusing a beam of radiation at an oblique incidence angle to illuminatea line on the surface, said beam and a direction through the beam andnormal to the surface defining an incidence plane of the beam; andimaging said line onto an array of detectors outside of the incidenceplane, each detector in the array detecting light from a correspondingportion of the line.

Yet another aspect of the invention is directed towards an apparatus fordetecting anomalies of a surface comprising means for focusing a beam ofradiation at an oblique incidence angle to illuminate a line on thesurface, said beam and a direction through the beam and normal to thesurface defining an incidence plane of the beam, said line beingsubstantially in the incidence plane of the beam; at least one array ofdetectors; and a system imaging said line onto the at least one array ofdetectors, each detector in the at least one array detecting light froma corresponding portion of the line.

One more aspect of the invention is directed towards an apparatus fordetecting anomalies of a surface and/or a surface feature, comprisingmeans for focusing a beam of radiation at an oblique angle to illuminatea line on the surface, said beam and a direction through the beam andnormal to the surface defining an incidence plane of the beam; at leastone array of detectors outside of the incidence plane; and a systemimaging said line onto the array of detectors, each detector in thearray detecting light from a corresponding portion of the line.

Yet another aspect of the invention is directed to an apparatus fordetecting anomalies and/or a surface feature on a first and a secondsurface of an object, comprising means for focusing a beam of radiationat an oblique incidence angle to illuminate a line on the first surface,said beam and a direction through the beam and normal to the firstsurface defining an incidence plane of the beam, said line beingsubstantially in the plane of incidence of the beam; at least one arrayof detectors; a system imaging said line onto the at least one array ofdetectors, each detector in the at least one array detecting light froma corresponding portion of the line; and means for detecting anomaliesand/or a surface feature of the second surface.

One more aspect of the invention is directed to an apparatus fordetecting anomalies and/or a surface feature on a first and a secondsurface of an object, comprising means for focusing abeam of radiationat an oblique angle to illuminate a line on the first surface, said beamand a direction through the beam and normal to the first surfacedefining an incidence plane of the beam; an array of detectors outsideof the plane of incidence; a system imaging said line onto the array ofdetectors, each detector in the array detecting light from acorresponding portion of the line; and means for detecting anomaliesand/or a surface feature of the second surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a surface inspection system toillustrate the preferred embodiment of the invention.

FIG. 2 is a top view of the system of FIG. 1.

FIG. 3 is a perspective view of the illumination portion of a surfaceinspection system to illustrate an alternative embodiment of theinvention.

FIG. 4 is a graphical plot of a point spread function useful forillustrating the operation of the systems of FIGS. 1 and 3.

FIG. 5 is a schematic view of a parallel array of charged coupleddevices (CCD) useful for illustrating the invention.

FIG. 6 is a schematic view of a light beam illuminating a line on asurface and corresponding positions of detectors of an array withrespect to an imaging system along the line 6-6 in FIG. 2 to illustratethe operation of the system of FIGS. 1-3 in response to height variationof the surface inspected.

FIG. 7 is a schematic view of the imaging optics, the CCD detectors anda portion of the surface to be inspected of the system of FIG. 1 takenalong the line 7-7 in FIG. 2 to illustrate the operation of the systemof FIGS. 1-3 in response to height variation of the surface toillustrate the invention.

FIG. 8 is a schematic view of the collection and imaging optics in thesystem of FIG. 1.

FIG. 9 is a perspective view of a portion of a wafer inspection systememploying a cylindrical mirror for illustrating another alternativeembodiment of the invention.

FIG. 10 is a schematic view of a system for inspecting the top andbottom surfaces of an object to illustrate another embodiment of theinvention.

FIG. 11 is a perspective view of the illumination portion of a surfaceinspection system to illustrate still another alternative embodiment ofthe invention.

For simplicity in description, identical components are labeled by thesame numerals in this application.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a perspective view of a surface inspection system toillustrate the preferred embodiment of the invention. System 10 includesa cylindrical objective such as a cylindrical lens 12 for focusing apreferably collimated light beam 14 to a focused beam 16 forilluminating, on surface 18 to be inspected, an area in the shape of aline 20. Beam 14 and therefore also focused beam 16 are directed at anoblique angle of incidence to the surface 18. Different from theapproach by Gara described above, line 20 is substantially in theincidence plane or plane of incidence of focused beam 16. In thiscontext, the incidence plane of beam 16 is defined by the common planecontaining beam 16 and a normal direction such as 22 to surface 18 andpassing through beam 16. In order for the illuminated line 20 to be inthe focal plane of lens 12, cylindrical lens 12 is oriented so that itsprincipal plane is substantially parallel to surface 18. Image of theline is focused by an imaging subsystem 30 to an array of detectors,such as a linear array of CCDs 32. The linear array 32 is preferablyparallel to line 20.

In one embodiment particularly advantageous for detecting small sizeanomalies, the imaging subsystem 30 has an optical axis 36 which issubstantially normal to line 20 so that the center portion of the linearCCD array 32 is in a plane substantially normal to the incidence planeof beam 16. The optical axis 36 may be oriented in any direction withinsuch plane, including a position directly above the line 20. In suchevent, array 32 would also be directly above line 20. If desired,another array 32′ shown in dotted line in FIG. 2 may be placed in aposition diametrically opposite to array 32, where array 32′ has opticalaxis 36′ also substantially normal to line 20. The two arrays togethermay be useful to detect 45 degree line patterns.

The imaging subsystem 30 projects an image of a portion of the line 20onto a corresponding detector in the CCD array 32 so that each detectorin the array detects light from a corresponding portion of the line 20.The length of the line 20 is limited only by the size of the collimatedinput beam 14 and the physical aperture of lens or lens combination 12.In order to control the length of line 20, an optional expander 34 shownin dotted lines may be used for controlling the diameter of beam 14 soas to control the length of line 20.

FIG. 3 is a perspective view of an illumination portion of a waferinspection system to illustrate an alternative embodiment of theinvention. To simplify the diagram, the portion of the system forcollecting and projecting an image of the illuminated line onto adetector array has been omitted. Instead of using a single symmetricallens, the embodiment in FIG. 3 employs two cylindrical lenses 12′ fortighter focusing, that is, focusing to a thinner line. In FIG. 1, boththe illumination and collection portions of system 10 are stationary andsurface 18 is rotated about a spindle 50 which is also moved alongdirection 52 so that line 20 scans surface 18 in a spiral path to coverthe entire surface. As shown in FIG. 3, the surface 18′ to be inspectedcan also be moved by an XY stage 54 which moves the surface along the Xand Y directions in order for line 20 to scan the entire surface. Again,the illumination and collection portions of system 10′ of FIG. 3 remainstationary. This is advantageous since it simplifies the opticalalignment in the system, due to the fact that there is substantially norelative motion between the illumination portion and the collectionportion of the system.

FIG. 4 is a graphical illustration of the point spread function offocused line 20 along the focused direction along any point of the line.As shown in FIG. 4, the point spread function of line 20 is Gaussian inshape, such as one which is produced if a 488 nm argon laser is used.Line 20 may also exhibit a varying point spread function along line 20with a peak at the center of line 20. In order to avoid the variation ofintensity along the line, it may be desirable to expand the beam bymeans of expander 34 to a longer length such as 10 mm and only use thecenter or central portion of the line, such as the central 5 mm of theline, so that power variation along the imaged portion of the line isinsignificant. By means of an appropriate aperture in the imagingsubsystem described below, it is possible to control the portion of theline imaged onto the array.

FIG. 5 is a schematic view of the linear CCD array 32. As shown in FIG.5, the array 32 has dimension d in a direction parallel to the line 20,and W is the illumination line width. In other words, the image of line20 as projected onto array 32 by subsystem 30 has a width of W. Thepixel size of the inspection system 10 is determined by the scan pitch pand the pixel size of the detectors in the array 32 in a directionparallel to line 20, or d. In other words, the pixel size is dp. Thus,assuming that the useful portion of the illumination line projected ontothe CCD array 32 has a length of 5 mm, and the illumination line width Wis 10 microns and array 32 has 500 elements with d equal to 10 micronsand the scan line pitch is 5 microns, the effective pixel size on thewafer is 5 microns×10 microns, assuming that the image of the line atthe array has the same length as the line. In practice, to avoidaliasing, at least two or three samples are taken in each direction(along line 20 and normal to it) per effective optical spot size on thesample surface. Preferably, reasonably high quality lenses such asquality camera lenses are used, such as ones having 5 mm field of view,giving a 30° collection angle.

From the above, it is seen that system 10 has high sensitivity, sincethe effective “pixel” size is 5×10 microns, which is much smaller thanthat of Stonestrom et al. At the same time, due to the fact that thewhole line of pixels on the surface 18 are illuminated and detected atthe same time instead of a single illuminated spot as in Stonestrom etal., system 10 also has acceptable throughput. As noted above, thelength of line 20 is limited only by the size of the collimated beam 14and the physical aperture of lens or lens combination 12. Thus, assumingthat the stage 54 has a stage speed of 10 microns per 0.1 millisecond,for a line scan rate of 10 kHz, the surface can be scanned at a speed of100 mm per second. For a line 20 of 5 mm, the wafer surface is thenscanned at a speed of 5 cm²/sec.

System 10 is also robust and tolerant of height variations and tilt ofsurface 18 and 18′. This is illustrated in reference to FIGS. 1, 2, 5-7.FIG. 6 is a cross-sectional view of a portion of the surface 18 alongthe line 6-6 in FIG. 2, focused beam 16 and two images of the array 32when the surface 18 is at two different heights. FIG. 7 is across-sectional view of the CCD array 32, imaging subsystem 30 and twopositions of a portion of the surface 18 to be inspected along the line7-7 in FIG. 2.

In reference to FIGS. 1, 2 and 6, the imaging subsystem 30 will alsoproject an image of the CCD array 32 onto surface 18 overlapping that ofline 20. This is illustrated in FIG. 6. Thus, if surface 18 is in theposition 18A, then imaging subsystem 30 will project an image 32A of thedetector array on surface 18A, as shown in FIG. 6. But if the height ofthe surface is higher so that the surface is at 18B instead, then theimaging subsystem will project an image of the detector array atposition 32B. The longer dimension of beam 16 is such that itilluminates both images 32A and 32B of the array.

From FIG. 6, it will be evident that the image of a particular detectorin the array will be projected on the same portion of the surface 18irrespective of the height of the surface. Thus, for example, theimaging subsystem 30 will project the first detector in the array 32 toposition 32A(1) on surface 18A, but to the position 32B(1) on position18B of the surface as shown in FIG. 6. The two images are one on top ofthe other so that there is no lateral shift between them. In the reverseimaging direction, an image of the same portion of surface 18 and,therefore, of line 20 will be focused to two different positions on thearray 32, but the two positions will also be shifted only in thevertical direction but not laterally. Hence, if the detectors cover bothpositions, then the variation in height between 18A, 18B of the surfacewill have no effect on the detection by array 32 and the system 10, 10′is tolerant of vertical height variations of the surface inspected.

One way to ensure that the array 32 covers the images of line 20 onsurface 18 at both positions 18A, 18B is to choose detectors in array 32so that the dimension of the detectors in the vertical direction is longenough to cover such change in position of the surface, so thatdifferent positions of a portion of the line 20 will be focused bysubsystem 30 onto the detector and not outside of it. In other words, ifthe vertical dimension of the detector is chosen so that it is greaterthan the expected height variation of the image of the line caused byheight variation of the wafer surface, the change in wafer height willnot affect detection. This is illustrated in more detail in FIG. 7.

As shown in FIG. 7, the pixel height (dimension normal to optical axisand line 20) of array 32 is greater than the change in position of theimage of line 20 caused by a change in wafer surface height, so that theimaging optics of subsystem 30 will project the same portion of thesurface and line on the wafer surface onto the same detector.Alternatively, if the pixel height of the CCD array 32 is smaller thanthe expected change in position of image of line 20 due to heightvariation in the wafer surface, multiple rows of CCDs may be employedarranged one on top of another in a two-dimensional array so that thetotal height of the number of rows in the vertical direction is greaterthan the expected height variation of the line 20 image. If this totalheight is greater than the expected movement of the image of the line inthe vertical direction, then such two-dimensional array will be adequatefor detecting the line despite height variations of the wafer surface.The signals recorded by the detectors in the same vertical column can besimply added to give the signal for a corresponding portion of the line20.

Even if the height or vertical dimension of array 32 is smaller than theexpected height variation of the wafer surface, the imaging optics ofsubsystem 30 may be designed so that the change in height or verticaldimension of the projected image of line 20 onto the CCD array is withinthe height of the CCD array. Such and other variations are within thescope of the invention. Thus, in order for system 10 and 10′ to betolerant of wafer height variation, the image of the line at the array32 is longer than the array, and the extent of the height variations ofthe image of the line 20 on the detector array is such that theprojected image still falls on the detector array.

Where a two-dimensional array of detectors is employed in array 32, timedelayed integration may also be performed to improve signal-to-noise orbackground ratio, where the shifting of the signals between adjacentrows of detectors is synchronized with the scanning of the line 20across surface 18.

FIG. 8 is a schematic view illustrating in more detail the imagingsubsystem 30 of FIGS. 1 and 2. Subsystem 30 preferably comprises twoidentical lenses: lens 102 for collecting light from line 20 and toperform Fourier transform, and lens 104 for imaging the line onto thearray 32. The two lenses 102, 104 are preferably identical to minimizeaberration. A filter and polarizer may be employed at position 106 whereline 20, position 106 and array 32 appear at focal points of the twolenses 102, 104 each having a focal length f. Arranged in this manner,subsystem 30 minimizes aberration. As noted above, a variable aperturemay also be applied at a number of positions in subsystem 30 to controlthe portion of the line 20 that is focused onto array 32 by controllingthe size of the aperture.

Instead of using a cylindrical lens 12 as shown in FIGS. 1 and 2, acylindrical mirror may be used as shown in FIG. 9. In order for line 20to appear in the focal plane of cylindrical mirror 112, the mirrorshould be oriented so that the plane 112′ defined by and connecting theedges 112 a, 112 b of the mirror is substantially parallel to surface 18inspected. In general, any cylindrical objective that has the effect offocusing a beam 14 onto a focused line on surface 18 may be used, wherethe focusing power is applied only in the direction substantially normalto the incidence plane defined by focus beam 16 and a normal 22 tosurface 18 through the beam.

An alternative method of generating a line focus on the sample is to usea cylindrical lens in the convention way, i.e. with its principal planeperpendicular to the propagation direction of the light beam 14, andplacing a diffraction grating 252 immediately following the lens. Thegrating period is such that main diffraction angle matches the desiredillumination angle range. The lens and the grating are held parallel toeach other, and to the sample surface 18. The grating line structure (orgrooves) are perpendicular to the focused line direction. The grating,therefore, will only have the effect of redirecting the light along thedesired incidence angle. Although a variety of different grating typescan be used, it is preferable to use a holographic type grating for itsenhanced efficiency.

By placing array 32 outside of the plane of incidence of beam 16 in adouble dark field configuration, signal-to-noise or background ratio isimproved over prior designs. A double dark field collector configurationis one where the optical axis of the collector in the subsystem isperpendicular to the optical axis of illumination and the collector liesoutside the incidence plane. However, in some applications, it may bedesirable to place the array in the incidence plane. Preferably, beam 16is at an angle in the range of about 45 to 85 degrees from a normaldirection to surface 18. In addition to detection of anomalies, theinvention can also be used to detect other surface features such asmarkers.

The invention as described above may be used to provide a viablealternate mechanism to inspect rough films, patterned or unpatternedsemiconductor wafers and backsides of wafers, as well as photomasks,reticles, liquid crystal displays or other flat panel displays. Thesystem of this invention is compact, has a simple architecture, andprovides a relatively low cost alternative for inspecting patternedwafers. Furthermore, because of the low cost of the system of thisinvention, it may also be advantageously used in conjunction withanother surface inspection system for inspecting two different surfacesof an object, as illustrated in FIG. 10. Thus, as shown in FIG. 10, asystem 200 may include a front side inspection system 202 for inspectingthe front side 204 a of the semiconductor wafer 204, and a system 206(which may be similar to that in FIGS. 1, 2 or 3) for inspecting thebackside 204 b of the wafer. If, as in the invention described above,the illumination and light collection portions of the system remainstationary and the surface 204 b is inspected by moving the surface, thetwo systems 202, 206 may need to be synchronized. System 202 may includea system such as that described above in reference to FIGS. 1-3, or maybe one of many different kinds of anomaly and surface feature inspectionsystems. All such variations are within the scope of the invention.

While the invention has been described by reference to variousembodiments, it will be understood that modification changes may be madewithout departing from the scope of the invention which is to be definedonly by the appended claims or their equivalents.

1. A method for detecting anomalies and/or features of a surface,comprising: focusing a beam of radiation at an oblique incidence angleto illuminate a line on the surface, said beam and a direction throughthe beam and normal to the surface defining an incidence plane of thebeam, said line being substantially in the plane of incidence of thebeam; and imaging said line onto an array of detectors, each detector inthe array detecting light from a corresponding portion of the line.